Glass fiberization method

ABSTRACT

Glass fibers for insulation uses are produced by means of a centrifugal spinner which introduces glass streams into an annular attenuating blast adjacent the periphery of the spinner. An improved product quality and/or production rate as well as prolonged spinner life are obtained by selection and utilization of a novel combination of structural and operating parameters characterized in particular by a spinner diameter and peripheral speed substantially greater than conventionally employed.

CROSS REFERENCE TO RELATED APPLICATION

This is a divisional of co-pending application Ser. No. 851,296, filed4/7/86, which is a continuation of Ser. No. 539,728, filed Oct. 6, 1983,which is a continuation-in-part of Ser. No. 461,834, filed Jan. 28,1983, both abandoned which is a continuation-in-part of Ser. No.409,336, filed Aug. 18, 1982 now U.S. Pat. No. 4,451,276 granted May 29,1984.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the fiberization of glass orother thermoplastic materials and relates more particularly tofiberization techniques wherein the molten material to be fiberized iscentrifugally converted by a rapidly rotating spinner into amultiplicity of glass streams which are attenuated into fibers by aconcentric annular gaseous blast from an internal combustion burneradjacent the periphery of the spinner directed perpendicularly to thecentrifugal stream, such a fiberization technique being herein referredto as "centrifugal blast attenuation". The fibers, after being sprayedwith a binder, are collected on a foraminous conveyor in the form of ablanket or mat, which is then passed through a curing oven.

2. Description of Prior Art

The centrifugal blast attenuation glass fiberization technique generallydescribed above has been used industrially for many years in theproduction of glass fiber insulation products, and a substantialpercentage of glass fiber insulation manufactured at the present time isproduced utilizing this technique. Details of various forms of thisprocess are disclosed for example in U.S. Pat. Nos. Re. 24,708,2,984,864, 2,991,507, 3,007,196, 3,017,663, 3,020,586, 3,084,381,3,084,525, 3,254,977, 3,304,164, 3,819,345 and 4,203,745.

In carrying out this technique, substantial amounts of heat energy arerequired, first for heating the glass into a molten state, and secondlyfor producing the attenuating blast. The uncertain availability and highcost of energy have created an increasing demand for glass fiberinsulation products, while the same factors have caused a substantialincrease in the cost of producing such products.

Efforts have accordingly been made to improve the efficiency of thedescribed fiberization process or to utilize alternate fiberizationtechniques. For example, some glass fiber production has in recent yearsbeen carried out utilizing a purely centrifugal fiber attenuation,primarily to avoid the energy requirements of the blast attenuationtechnique. Such a process is disclosed for exampl.e in U.S. Pat. No.4,058,386.

Centrifugal stream formation coupled with blast attenuation as generallydescribed above remains a preferred technique however, both because ofthe excellent quality of the fiber blanket obtained therewith as well asthe fact that a substantial portion of the insulation industry isequipped at present with apparatus for carrying out such a process. Itaccordingly follows that any improvement in this technique would be ofsignificant industrial importance. As will be understood from thefollowing disclosure, the present invention provides marked improvementsin centrifugal blast attenuation fiberizing techniques with respect toproduct quality, production rate, and operating costs.

Inasmuch as glass fiberization is in practice an extremely complextechnique characterized by a large number of variable parameters, manyof the details of known techniques need not be included herein,reference being made to the above patents for such disclosures. However,certain limited aspects of the prior art will be considered, especiallyconcerning those factors respecting which the present invention departssubstantially from prior practice.

Among the many variables to be considered, the construction of thespinner and the speed at which it rotates are of particular importancein successfully carrying out a centrifugal fiberization process. Inaddition, the diameter of the spinner, the size, number and arrangementof the orifices in the peripheral spinner wall, the alloy from which thespinner is made as well as the shape of the spinner wall, thedistribution of molten glass to the interior spinner wall and thecontrol of the temperature of various portions of the spinner assemblyand the glass flowing therewithin are factors which must be carefullyconsidered.

In the centrifugal blast attenuation process, the blast temperature andvelocity, as well as the placement of the blast nozzle and direction ofthe blast with respect to the spinner wall are important to anoptimization of the fiber attenuation. Spinner life is an importantfactor, particularly in view of the relatively short life of this typeof spinner and the extremely high cost of spinner replacement.

The spinners used in early centrifugal blast attenuation equipment weretypically of a diameter of about 200 mm and the peripheral wall thereofincluded typically 4,000 to 6,000 holes through which the molten glasspassed to form the primary glass streams subjected to attenuation by theannular blast. It was perceived at an early date that for a spinner ofgiven size and construction, the output or pull rate, conventionallyexpressed in terms of the weight in tons per day of produced fiber,could be increased only at the expense of a corresponding decrease infiber quality. It was further perceived that there were practical limitsto the pull rate per spinner orifice for maintaining acceptable fiberquality, the maximum rate per orifice ranging between about 1 and 1.4Kg/day. Nonetheless, the economic demands for increasing production of agiven line usually resulted in an increase in pull rate despite thedeterioration in product quality. The term "quality" in this senserefers to the product weight per unit of area for a given thermalresistance and nominal product thickness. A lower quality product wouldhence be a heavier product although with the same insulating value asthe better quality product. The lower quality product is thus lower inquality not only since it has a higher density, but also in the sensethat it is inherently a more expensive product, requiring more glass fora given area, and is thus more costly to manufacture.

In an effort to increase the output of a spinner of given diameter, thenumber of holes in the peripheral wall of the spinner was increased.Although some increase in pull rate was achieved, there are practicallimits of orifice density controlled by factors such as the necessity ofmaintaining discrete glass streams emerging from the periphery of thespinner and manufacturing problems. Similar considerations limit thedegree to which the spinner peripheral wall can be increased in heightto increase its area.

Since the pull rate per orifice, orifice density, and height of thespinner wall could not be further increased without sacrificing fiberquality below acceptable limits, efforts to increase the pull rate weredirected toward increase of the spinner diameter, initially to 300 mmand more recently to 400 mm. Although each such increase in diameterproduced some increase in pull rate and/or an improvement in fiberquality, the improvements were modest in comparison with those of thepresent invention.

Another limiting factor is the centrifugal acceleration produced by thehigh rate of spinner rotation. Although substantial centrifugal forcesare required to produce the necessary flow of molten glass through thespinner orifices and to thereby form the primary glass streams, highcentrifugal forces foreshorten the life of the spinner.

Since spinner life is substantially inversely proportional to thespinner centrifugal acceleration forces, it has heretofore beenconsidered desirable to restrict rotational speeds of the spinner asmuch as possible in an effort to extend the spinner life.

Due to the detrimental effects of higher centrifugal acceleration onspinner life and the uncertain effects of higher peripheral speeds onfiber attenuation, the conventional wisdom when increasing spinnerdiameter has been to decrease or refrain from increasing the centrifugalacceleration and to hold peripheral velocity within a range known togive satisfactory attenuation.

A further factor is of importance, namely the fineness (averagediameter) of the fibers. It is well established that for a given densityof fiber mat layer, the finer the fibers, the greater the thermalresistance of the layer. An insulating product comprising finer fiberscan accordingly be thinner with the same insu1ating value as a thickerproduct of coarser fibers. Or, likewise, a product of finer fibers canbe less dense than one of coarse fibers of the same thickness and havethe same insulating value.

Since sales of insulation products are usually based on a guaranteedthermal resistance (R value) at a nominal thickness, the fiber finenessis an important factor determining the weight of the product per unit ofarea, known as the basis weight, a product of finer fibers having thelower basis weight and hence requiring less glass and enjoyingmanufacturing economies.

From an economic standpoint, however, fiber fineness, as with otherfactors, is normally considered to be a compromise since the attainmentof finer fibers is thought to flow principally from higher blastvelocities and from the use of softer glass compositions. Increasing theblast velocity results in a direct increase in energy costs, and softerglasses typically require ingredients which are expensive and which,further, usually have undesirable pollutant characteristics.

Fineness, which can be expressed in terms of fiber diameter, in microns,representing the arithmetic mean value of measured fiber diameters, isalso conveniently expressed on the basis of a fiber fineness index, or a"micronaire" determination, the latter being a standard measuringtechnique in the glass wool industry wherein a predetermined mass orsample, for example 5 grams of the fibers, is positioned within ahousing of a given volume so as to form a permeable barrier to airpassing through the housing under a predetermined pressure and themeasurement is made on the air flow through the sample. The measurementthus made depends on the fiber fineness.

In general, the finer the fibers the more resistance offered to thepassage of air through the sample. In this manner an indication is givenof the average fiber diameter of the sample. The fineness of typicalblast attenuated centrifugal glass fiber insulation products ranges fromfine types (i.e. micronaire 2.9 (5 g); average diameter 4 μm) torelatively coarse types (i.e. micronaire 6.6 (5 g); average diameter 12μm).

The insulating value of a blanket of fibers is dependent to a limitedalthough significant degree on the lay-down of the fibers on thecollecting conveyor, which determines the orientation of the fibers inthe insulation product. The thermal resistance of a fiber blanket willvary depending on the direction of orientation of the fibers to themeasured heat flow, the resistance being greater when the fibers areoriented perpendicular to the direction of heat transfer. Accordingly,to maximize the thermal resistance of an insulating blanket, the fibersshould be oriented to the maximum degree possible in an attitudeparallel to the collecting conveyor and the plane of the blanket formedthereon. Because of the extreme turbulence generated above thecollecting conveyor by the decelerating fibers and gaseous currents,there is very little that can be done to control the orientation of thefibers, most efforts in this area of the fiberizing process beingdirected toward achieving a relatively uniform distribution of fibersacross the width of the conveyor.

In an effort to increase fiber production still further whilemaintaining or possibly improving the quality of the fibers, experimentswere undertaken with still larger spinners having diameters of 600 mmand over. Surprising improvements in pull rate and/or quality wereachieved although for reasons some of which at the present time are notentirely clear. Particularly unexpected were improvements in the fiberquality which exceeded forecasts by 10-25% depending on the pull rateutilized.

In making the transition to the large size spinner, the spinnerrotational speed was reduced to provide centrifugal accelerating forceson the glass and spinner wall within conventional limits such that theglass feed through the orifices, as well as the stresses placed on thespinner wall structure, would not depart significantly from priorpractice. It was feared that the larger spinner diameter would, at therotational speed necessary to produce such centrifugal force, result inan unacceptably high peripheral speed of the spinner with a consequentdegradation of fiber formation and attenuation. Surprisingly, thefiberization was not adversely affected by the higher peripheral speed,but, to the contrary, was actually improved as evidenced by the improvedfiber quality and/or pull rate as compared with lower peripheral speeds.Furthermore, it has been found that the larger spinner diametersignificantly improves the operating economy of the system.

BRIEF SUMMARY OF THE INVENTION

The present invention in summary comprises techniques including bothmethod and apparatus for producing glass fibers, and fibrous insulationblanket or mat made therefrom.

The present method contemplates a peripheral spinner velocitysubstantially higher than heretofore conventionally employed andpreferably in the range of about 50 m/s to about 130 m/s.

The apparatus includes means for supplying a stream of molten glass tothe spinner for centrifugal delivery to the interior surface of theperipheral spinner wall. This wall includes a plurality of orificesthrough which the molten glass passes and forms a multiplicity ofstreams. The invention contemplates formation of fibers from thecentrifugally delivered streams by the use of gas blast attenuation.

The technique of the invention includes an internal combustion burnerproviding a downwardly directed annular blast adjacent the spinner wallto attenuate the glass streams into fibers. The fibers are delivereddownwardly into a receiving hood or receiving chamber and are collectedon a substantially horizontal foraminous conveyor disposed at the bottomof the receiving chamber

Although the improved fiberization achieved with the higher spinnerperipheral speed is obtainable with spinners of conventional size, ithas been found that a substantially increased pull rate and henceincreased operating economies can be achieved with a substantialincrease in spinner diameter. It is accordingly preferred that higherspinner peripheral speed be employed in conjunction with a spinner ofincreased diameter in carrying out the present invention. To achieveimproved operating economies, the spinner diameter should besubstantially greater than 500 mm and preferably within the range offrom about 600 mm to 1500 mm.

The invention is further directed to a glass fiber insulation blanket ormat having improved insulation quality, in addition to other improvedmechanical properties.

Although efforts have been undertaken to isolate the principal factorsresponsible for the improved performance obtained with the higherspinner peripheral velocity and the larger spinner diameter, at thepresent time these efforts have not been conclusive. Such theoreticalexplanations as are set forth in this specification must be recognizedto be tentative, subject to further experimental verification, and nottaken to be limiting.

The following disclosure will present a detailed description of theapparatus and process utilized in obtaining the improved performance aswell as a description of the improved product obtained thereby.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial sectional elevational view showing a spinnerassembly and burner in accordance with the present invention;

FIG. 2 is a schematic elevational view showing the operation of aconventional small diameter spinner and fiber collecting conveyor, theview being taken transversely through the conveyor and illustrating theuneven distribution and random orientation of fibers on the conveyor inthe absence of fiber distribution means;

FIG. 3 is a view similar to FIG. 2 but employing a large diameterspinner operating in accordance with the invention showing therelatively uniform distribution and relatively uniform orientation offibers on the conveyor in the absence of fiber distribution means;

FIG. 4 is a schematic plan view showing a plurality of spinners andtheir arrangement with respect to the conveyor;

FIG. 5 is a schematic elevational view of the apparatus shown in FIG. 4;

FIG. 6 is a graph showing fiber quality plotted against spinner diameterfor fibers of various degrees of fineness;

FIG. 7 is a graph showing energy consumption plotted against spinnerdiameter for fibers produced at constant centrifugal acceleration;

FIG. 8 is a diagrammatic perspective view of a spinner and the path ofthe fiber formed at the periphery thereof; and

FIG. 9 is a view illustrating the projection of the path of the fiber inFIG. 8 into the plane tangential to the spinner.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawings and particularly FIG. 1 thereof, a fiberizingstation in accordance with the present invention is illustratedincluding a spinner 10 having a peripheral wall 12 and a support plate18. The spinner 10 is mounted by means of a hub portion 20 to asubstantially vertical shaft 22. The shaft 22 is rotatably supported ina well known manner by suitable bearings attached to a supporting frameand is driven in rotation at a relatively uniform predetermined speed byan electric motor and belt drive. The shaft support and drive detailsare conventional and accordingly are not illustrated.

The shaft 22 is hollow, permitting a stream of molten glass 24 to passdownwardly therethrough into a basket 26 supported beneath the lower endof the shaft by bolts 32.

The basket 26 comprises a substantially cylindrical wall 34 having aplurality of orifices 36 through which the molten glass passes under theinfluence of centrifugal force in streams 38 which are directed onto theinterior of the spinner wall 12. A multiplicity of orifices 40 in theperipheral wall 12 of the spinner serve to form a multiplicity of moltenglass streams 41 as the molten glass is forced through the orifices bythe centrifugal force acting thereon.

As discussed hereinafter, the diameter of the spinner, the size anddensity of the orifices 40, as well as the speed of rotation of thespinner are parameters important to the fiberizing process.

An annular internal combustion burner 42 of substantially conventionalconstruction is disposed above the wall of the spinner and includes anannular blast nozzle 44 spaced above the spinner peripheral wall 12 soas to direct an annular blast downwardly adjacent the spinner wall 12 tointercept and attenuate the multiplicity of glass streams 41 issuingfrom the orifices 40. The burner 42 includes a metal casing 46enveloping a refractory liner 48 defining an annular combustion chamber50 into which an air-fuel mixture is introduced at inlet 52. The blastnozzle 44 communicates with the combustion chamber 50 and is formed byinner and outer nozzle lips 54 and 56. The blast nozzle lips 54 and 56respectively include internal cooling channels 54a and 56a into which acooling liquid such as water is introduced by inlet 60 for circulationto an outlet (not shown).

In order to maintain the heat content of the spinner and fibers duringattenuation, a high frequency induction heating ring 62 is provided justbelow the spinner in concentric relation thereto and having an internaldiameter somewhat larger than the spinner to avoid interference with thedownward flow of fibers entrained by the annular blast.

An auxiliary blast is generated by an annular blowing crown 64 disposedoutboard of the blast nozzle lips and connected to a source ofpressurized gas such as air, steam or combustion products.

The hollow shaft 22 includes several fixed concentric internal tubes.The innermost pair of these tubes defines an annular cooling passage 66through which cooling water is circulated while the outermost pairdefine an annular passage 68 through which a combustible mixture can bepassed and ignited to preheat the basket 26 prior to startup of thespinner.

The fibers generated by the spinner and the gaseous blast passdownwardly into a receiving chamber or receiving hood 70 and are thencedeposited in the form of a blanket 71 on a foraminous conveyor 72 asshown schematically in FIGS. 2, 3 and 5. A suction box 74 beneath theconveyor withdraws the high volume of gases passing through the conveyorin a conventional manner.

As shown in FIGS. 4 and 5, a plurality of fiberizing stations eachhaving a spinner 10 are conventionally employed for the production ofthe blanket 71 and in the preferred form of the invention are arrangedin a line along the longitudinal axis of the conveyor 72. The number ofspinners directing fibers onto a conveyor in an industrial installationmight typically be six to ten spinners or more.

For operation of the described apparatus, the spinner 10 including thebasket 26 thereof is preheated in a well known manner utilizing thecombustion of gases passing through passage 68, the heat of the burner50 and heating ring 62 and similar supplemental sources as may benecessary.

With the spinner rotating at a predetermined speed and the burneradjusted to provide a combustion chamber pressure resulting in a blastvelocity sufficient to provide the desired attenuation and fineness ofthe fibers, the molten glass stream 24 is introduced into the hollowspinner shaft 22 from a forehearth or other source of molten glassdisposed above the spinner assembly. The stream of molten glass uponreaching the basket 26 flows along the bottom of the basket under theinfluence of centrifugal force and passes through the orifices 36 of thebasket in the form of glass streams 38 which are directed onto the upperportion of the spinner peripheral wall 12.

Under the influence of the stronger centrifugal force exerted at thewall 12, the glass passes through the multiplicity of small orifices 40and issues at the exterior of the peripheral wall in the form of amultiplicity of streams 41 which are immediately subject to theattenuating effect of the blast from the internal combustion burner 50directed across the exterior of the wall. The glass streams 41 aremaintained in an attenuable condition by the elevated temperature of theblast for a time sufficient to effect attenuation thereof. The finenessof the attenuated fibers is regulated primarily by the control of theblast velocity which in turn is a function of burner pressure. Anincrease in burner pressure and blast velocity will result in a greaterattenuation and hence finer fibers.

The flow of attenuated fibers into the receiving chamber or receivinghood 70 as shown in FIGS. 3 and 5 is accompanied by the induction ofsubstantial amounts of air as shown by the arrows at the top of thereceiving chamber. Although the induced air tends initially to restrictthe expansion of the veil of fibers flowing from the spinner, the rapiddeceleration of the fibers within the receiving chamber produces asubstantial expansion of the fiber veil and, for reasons discussed inmore detail herebelow, provides a relatively uniform distribution of thefibers amid the product and across the width of the conveyor.Furthermore, due to a diminution of the turbulence usually present inthe conveyor region, the invention produces a more favorable orientationof the fibers during the formation of the fiber blanket with a resultantimprovement of the thermal properties of the blanket.

Although a binder spray is applied to the attenuated fibers at the topof the receiving chamber in a conventional manner, the showing of theapparatus for applying the binder has been omitted in FIGS. 2-5 tosimplify these figures.

The spinner peripheral velocity and the diameter of the spinner areimportant factors in the present technique.

The largest spinners in industrial use in centrifugal blast attenuatedprocesses have heretofore had a diameter on the order of 400 mm and aperipheral velocity of approximately 44 m/s. An increase in spinnerdiameter and peripheral velocity had not been deemed feasible, even ifcentrifugal acceleration were not increased. Peripheral velocityincreases were thought to present difficulties in fiber attenuation. Inaddition, difficulties with the life of the spinner were anticipated.

It has been discovered, however, that substantial increases in spinnerdiameter and peripheral velocity have no adverse effects onfiberization, and in fact, produce fiber of improved quality whenoperated at the same pull rate per spinner as the 400 mm sized spinner.For example, a spinner of 600 mm in accordance with the presentinvention can be operated at a pull rate about 50% higher than a 400 mmspinner while producing the same quality fiber. The economic advantagesof such an improvement are evident, particularly when it is consideredthat the output of a given production line can be increased by at least50% utilizing the invention with modifications requiring a capitaloutlay in a typical situation of less than 3% of the cost of a newproduction line.

Considering further the factor of spinner diameter, excellent resultshave been achieved utilizing a spinner of 600 mm diameter andsubstantially larger spinners can be used. The benefits of the inventioncan be attained with spinners having a diameter substantially in excessof 500 mm and within the range of about 550 mm to about 1500 mm. Thepreferred range of spinner diameter is 600 mm to 1000 mm.

It has been found that fiberization and hence fiber quality is generallyimproved with increased peripheral speed and increased centrifugalacceleration, although the latter is detrimental to spinner life. Byselecting a spinner rotational speed providing centrifugal accelerationforces not significantly departing from those conventionally utilized insmaller spinners, for example within the range of about 8,000 to 14,000m/s², the peripheral speed with the larger size spinners would besignificantly higher than that conventionally employed with smallerspinners with a resultant improved fiber quality. The spinner life wouldnot be decreased, the larger spinners experiencing substantially thesame centrifugal forces as the smaller conventional spinners. Forexample, with a 600 mm spinner operating at a speed producing acentrifugal acceleration of 10,600 m/s², the peripheral speed would be56.5 m/s, substantially higher than the peripheral speed of 46 m/s ofthe conventional 400 mm spinner operating at the same centrifugalacceleration.

The present invention contemplates a rotational speed of the spinnerwhich, taking account of the preferred range of spinner diameters asdescribed above, would produce a centrifugal acceleration at the spinnerperipheral wall within the range of about 4,000 to about 20,000 m/s² anda peripheral speed ranging substantially between about 50 and about 130m/s. It is expected that the centrifugal acceleration would in practicerange between about 6,000 to about 16,000 m/s², particularly in view ofthe improvement in fiber quality noted within this latter range. Sincefiber quality improves with increasing centrifugal acceleration andperipheral speed, the only detriment to operating toward the upper endof the above ranges is the reduced spinner life.

Although in the preferred form of the invention, increased spinnerperipheral velocity is employed in conjunction with spinners ofsubstantially larger diameter, the higher spinner peripheral velocity isbeneficial even with spinners of conventional size in achieving improvedfiberization. Similarly, spinners of large diameter in accordance withthe invention can produce operating economies and/or an improved qualityfiber even when operated at conventional speeds.

The improvements flowing from the larger spinner diameter will next beconsidered.

The graph of FIG. 6 illustrates the operation of spinners of varioussizes, at a substantially constant centrifugal acceleration of about10,000 m/sec². It is to be noted that fiber quality, for fiberfinenesses of 2.5, 3.0, 3.5 and 4.0 (under 5 grams), significantly andsharply improves, as is graphically shown by the distinct change in theangle of curvature, at spinner diameters substantially in excess of 500mm. Since the curves begin to increase substantially only above 400 mm,the improvements would not have been predictable from the resultsobtained with prior art.

The improvement in fiber quality is particularly substantial with theuse of large spinners in making fine fibers. For a fiber having amicronaire of 2.5 (5 g), for example, it has been confirmed that bychanging from a 400 mm spinner to a 600 mm spinner, a reduction inweight of the order of 5% can be attained.

Another factor having an important bearing on fiber production is theburner pressure, the control of which directly affects the fiberfineness and on which also depends the energy consumption of theprocess.

Utilizing a burner of the type shown in FIG. 1, the burner pressureshould range between about 100 and about 900 mm water column with apreferred pressure range of 200 to 600 mm water column. For reasons nottotally understood, the required burner pressure necessary to produce afiber of a certain fineness decreases with increasing spinner diameter,even though the centrifugal acceleration is not increased. This factoris considered to be the cause of the improved fiber quality as well asthe energy saving noted with the larger spinners since the lower burnerpressure results in longer fibers with less fiber breakage.

The results of the research carried out by the inventors on thissubject, and which is summarized in FIG. 7, shows a great decrease inthe energy consumed when the diameter of the spinner increases. Thecurve shown on this graph is for a constant acceleration of 10,000 m/s².In these experiments the density of the orifices on the spinner walls,the dimensions of the orifices, the pull per orifice and the fineness ofthe fibers produced are identical.

The graph of FIG. 7 corresponds to the production of very fine fibers,micronaire 3 (5 g). It is established in particular that under theconditions of the invention, in other words with the spinners having adiameter greater than 500 mm, the consumption of heat is less than 1500kcal/kg whereas it is for example 1750 kcal/kg with fibers producedusing a spinner with a diameter of 300 mm.

Since the heat consumption for the formation of the gaseous attenuatingblast represents most of the energy consumed in producing the fibers, atleast 4/5ths of the total, reduction in this consumption has a verysubstantial effect on the fiber production cost.

Although the thermal consumption values correspond to the same qualityof fibers, the quantities produced are not the same. Thus, forconditions which allow the same quality of fibers to be obtained,spinners 300, 400, and 600 mm in diameter produce respectively 10, 13.5and 20 metric tons of fibers per day. The economies in consumption aretherefore in addition to the economies provided by the increase inproduction rate.

The width of the burner nozzle 44 preferably is within the range ofabout 5 mm to about 20 mm with a preferred width of about 8 mm. Theburner temperature preferably ranges between about 1300° C. and 1700° C.with a preferred temperature of about 1500° C.

With spinners contemplated by the present invention, it has been foundthat an improved distribution of the fibers on the conveyor as well asan improved orientation of the fibers within the blanket can beobtained.

Measurements were made on products obtained according to the priortechnique and products obtained with the technique of the presentinvention.

There are several methods of measuring the fiber distribution within theinterior of a product. One of the simplest ones involves cutting theproduct into a series of small parallelipipeds or "cubes" (for exampleof 25×25×45 mm in size) which are individually weighed. The differentweights, which can be expressed in local densities related to the centerof gravity of each "cube", give a three dimensional picture of thedistribution. To facilitate the comparisons the coefficient of variationC_(v) of the distribution is calculated by the quadratic differential(square root of the mean value of the differentials squared) to the meanvalue of the weight of the "cubes".

Utilizing this method of measuring distribution, a very significantdifferential was found between a product obtained with the priortechnique (C_(v) =6.1%) and that obtained with the technique of thepresent invention (C_(v) =2.6%). This shows a very substantialimprovement in the fiber distribution in the products prepared inaccordance with the invention.

In the example of the invention illustrated in FIG. 3, the relativelylarge diameter of the spinner results in a veil of fibers which expandsbefore reaching the conveyor and the width of which is greater than thewidth of the conveyor, the fibers around the edge of the veil at eachside of the conveyor encountering the sides of the receiving hood 70 andbeing redirected inwardly to produce a blanket 71 of relatively uniformthickness. The lay-down of the fibers occurs with a minimal amount ofturbulence and accordingly results in a fiber orientation predominatelyparallel to the direction of the conveyor.

In contrast, an example of the prior art is shown in FIG. 2 wherein thefiber veil is seen to be too narrow to reach the walls of the receivinghood and, as a result, due to the typical concentration of fibers in thecenter of the veil, the blanket is nonuniform, being disproportionatelythick in the center and thin at the sides. Furthermore, in contrast tothe lay-down of the fibers with the larger spinner shown in FIG. 3, asubstantial turbulence occurs around the edge of the veil proximate theconveyor, which turbulence results in a disorganized lay-down of thefibers, the fiber orientation being substantially less parallel to theconveyor than that produced with the present apparatus and method.

Because of the poor distribution attainable with the conventionalsmaller spinners operating under conventional parameters, variousauxiliary means have been employed in an effort to improve the fiberdistribution

With the wide conveyors it is possible to place transversely two, threeor more fiberizing units in a transverse direction to the conveyor.However, even if theoretically this arrangement enables a uniformdistribution, it presents the major disadvantage that for any stoppageof a single unit of the row, for example to change the spinner, thedisorganization of the distribution resulting from this stoppage leadsto the rejection of the product formed by all the other units during thespan of the intervention. For this reason it is generally preferable toarrange the fiberizing units in a single line longitudinally to theconveyor, since, in this case, any stoppage of a unit would notappreciably alter the distribution and it would be possible to continueto produce, although at a lower rate.

With the units arranged in this manner, various types of auxiliarydistribution means are employed in an effort to improve the fiberdistribution. These distribution means include for example jet nozzlesin the side of the receiving hood (U.S. Pat. No. 3,030,659), oscillatingor alternately fed blower rings, baffles controlling induced air (U.S.Pat. No. 3,255,943), oscillating conduits for the fiber veils (U.S. Pat.No. 3,830,638) and the oscillation of the spinner assembly (U.S. Pat.No. Re. 30,192). Although such devices may achieve an improved fiberdistribution, they generally introduce even more turbulence into thereceiving chamber, thereby causing an even less favorable orientation ofthe fibers in the blanket. Since the fiber orientation is extremelyimportant in a fiber insulating medium, with a fiber orientationparallel to the conveyor providing improved thermal resistancecharacteristics, it can be understood that the larger veil produced bythe large spinners in accordance with the invention is an importantfactor in optimizing the quality of the fibrous blanket. Furthermore,the expense of auxiliary distribution devices and the cost of theiroperation can be minimized or eliminated with the present invention.

The shape of the veil of fibers directly beneath the spinner can be seento be more favorable in FIG. 3 than in FIG. 2, the veil in FIG. 3 havingrelatively little contraction beneath the spinner whereas that of FIG. 2is substantially contracted in this region. The cause of thisimprovement is not as yet known but may be a result of the increasedspinner peripheral speed which in some manner counteracts theconstricting effect of the induced air.

Although it is expected that the present apparatus and method can beeffectively utilized with any of the glass compositions conventionallyemployed for producing glass fibers by centrifugal blast attenuation,the glass composition preferably falls within the following ranges:

    ______________________________________                                        SiO.sub.2            61 to   72%                                              Al.sub.2 O.sub.3     2 to    8%                                               Fe.sub.2 O.sub.3     0.2 to  1%                                               CaO                  4.7 to  7.5%                                             MgO                  0 to    5%                                               Na.sub.2 O + K.sub.2 O                                                                             14 to   18%                                              B.sub.2 O.sub.3      0 to    6%                                               F                    0 to    1.5%                                             BaO                  0 to    2.5%                                             ZrO.sub.2            0 to    2.5%                                             Misc.                ≦1                                                ______________________________________                                    

The heat transfer characteristics of a fibrous material are usuallyexpressed in terms of its apparent conductivity which is derivedessentially from the sum of the conduction of the gas contained in thematerial, the solid conduction of the fibers and the radiation throughthe material. For fibrous insulation materials used in a confined spacesuch as blankets of fibrous material used as building insulation, forthe temperature ranges encountered, the heat transfer by convection isnegligible and can be ignored. The apparent thermal conductivity canthus be expressed as follows:

    λ=A+B(ρ)+C/ρ

where

λ=apparent conductivity W/m°K.

A=conductivity of the gas

B=Coefficient of conductivity related to the solid part of the fibrousmaterial

C=fiber surface area factor

ρ=density of the fibrous material.

We have found that in the carrying out of the invention, suitable valuesfor these factors are as follows:

A=25.89×10⁻³

B=0.02×10⁻³ to 0.2×10⁻³, preferably 0.075×10⁻³

C=0.100 to 0.300, preferably 0.190

ρ=8 to 75 kg/m³

The apparent conductivity for fibrous insulation materials typicallyranges between 30×10⁻³ to 55×10⁻³ at 24° C. with the fiber finenessranging between about 2 (5 g) to about 5 (5 g).

Examples of different methods of operating in accordance with theinvention are shown in the following table. Example I, which does notcorrespond to the conditions of the invention, is for comparisonpurposes. Examples II, III and IV show operations with spinner diametersof 600, 800 and 1000 mm respectively.

    ______________________________________                                                       Example No.                                                               I      II       III      IV                                        ______________________________________                                        Spinner Diameter                                                                           400      600      800    1000                                    (mm)                                                                          Pull rate per spinner                                                                      20       20       20     20                                      (metric tons/day)                                                             Burner nozzle width                                                                        7.7      7.7      7.7    6.5                                     (mm)                                                                          Burner Pressure (mm                                                                        430      350      400    420                                     water)                                                                        Burner temperature                                                                         1500     1500     1500   1500                                    °C.                                                                    Fineness (micronaire                                                                       4.2      3.5      3.0    2.5                                     under 5 g)                                                                    Density g/m.sup.2 for R =                                                                  1180     990      880    720                                     2 at 297° k.                                                           Nominal Thickness                                                                          80       90       90     90                                      (mm)                                                                          ______________________________________                                    

These examples show that for a same pull rate of 20 metric tons per day,the products obtained according to the invention are substantiallysuperior.

In comparing Examples I and II, it can be seen that for the same thermalresistance, the density is smaller according to the invention and alsothe burner pressure and thus the consumption of energy are also less.

Example III is analogous to Example II with a larger spinner diameter.The fineness and the density are still further improved.

Example IV is another example in which the dimension of the spinner isstill further increased. The fineness and the density of the productsobtained according to this example are particularly low; in other words,the fibers are very fine and the mass of fibers necessary to obtain agiven degree of thermal insulation is greatly reduced.

As indicated above, it has been found, contrary to expectations, thatincreased spinner peripheral speed as a factor independent of spinnerdiameter produces a substantial improvement in fiber mat quality andmechanical properties To achieve these improved results, the peripheralvelocity of the spinner should range between about 50 m/s and about 150m/s and preferably between 50 and 90 m/s.

There has been no satisfactory explanation why it is possible to improvethe quality of the products by using a high peripheral speed. It is wellknown that the attenuation of filaments of material passing fromorifices at the periphery of the spinner is a complex phenomenoninvolving the movement of the spinner and of the stream of gas whichentrains the fibers. It is impossible to determine with certainty therole of each of these factors in the fiber attenuation.

It can reasonably be assumed, however, that the fiber is attenuated by aprocess that can be likened to mechahical drawing, the filament beingheld at one "end" by the centrifuge and at the other "end" by thefriction exerted by the moving gases. In this theory, the relativemovement of the two "ends" of the fiber is the result of the combinationof the rotary movement of the spinner and the movement of the stream ofgas.

As shown in FIG. 8, the path F followed by the filaments on leaving thespinner 10 does appear to show that such a combination of movements isinvolved in the drawing process. Projection of the path F of thefilament into the plane T tangential to the spinner forms an angle withthe direction of the gas flow (which is usually parallel to the spinneraxis), and it will be found that this angle is in fact a function of theratio of the peripheral speed of the spinner to the speed of the gases.This is shown in FIG. 9 wherein point 0 represents a spinner orifice, OPrepresents the peripheral velocity of the spinner, OG represents the gasvelocity, OF the path of the filament (the resultant of vectors OP andOG) and α the angle of the filament with respect to the direction ofpropagation of gas flow.

Under conventional operating conditions, the angle α is relativelysmall, on the order of 20° or less, since the speed of the gases is muchhigher than the peripheral speed of the spinner. Accordingly, anincrease in the speed of the spinner should not have a significanteffect on the intensity of the fiber attenuation as shown in the brokenline OF' in FIG. 9.

Purely geometric considerations should lead to the conclusion that inorder to improve drawing of the fibers on the basis of conventionaloperating conditions an increase of the speed of the gases would be thebest solution as shown by the OF" in FIG. 9.

In fact, contrary to expectations, it appears that, with the otherconditions unchanged, when the speed of the gases is increasedsubstantially in order to try to obtain extremely fine fibers, productsare obtained whose qualities are substantially less satisfactory, and inparticular these fibers are very irregular, short and fragile while theinsulating blankets or mats prepared with these fibers have poormechanical and (to some extent) thermal properties.

Furthermore, it has surprisingly been found that very substantialimprovements in the quality of the insulating products are obtained ifthe peripheral speed is increased, all other conditions remainingunchanged.

Although it is desirable to have a high peripheral speed, it is obviousfor technical reasons that this speed cannot be increased indefinitely.In practice it is difficult to obtain speeds above 150 m/s and the speedaccording to the invention is preferably between 50 and 90 m/s.

The peripheral speed is particularly limited by mechanical strengthconsiderations of the spinners used Although periodic replacement of thespinner is inevitable, the life under the operating conditions accordingto the invention must remain compatible with the requirements oftechnical reliability and cost, which are inseparable factors in anyindustrial undertaking.

Replacement of the spinner under conventional operating conditions isusually necessary after several hundreds of hours of operation. It isimportant that the spinner life under the operating conditions accordingto the invention should not be shorter than that with the prior artprocesses and, if possible, even substantially longer.

A limitation of the speed of rotation--and hence limitation of thestresses experienced by the spinner--without obstructing theabove-identified conditions, therefore appears to be particularlydesirable for the use of the invention. Taking into account thematerials conventionally used, it would appear advantageous not toexceed the limit of 8,000 rpm.

Although the speed of rotation is limited to prevent the spinner frombeing subjected to excessive stresses, the centrifugal effect must stillbe adequate to project the material to be formed into fibers from thespinner orifices under satisfactory conditions in respect of output. Itis therefore preferable for the speed of rotation to be no less than 800rpm. The preferred range of rotation speed is between 1000-2250 rpm.

In practice with regard to the choice of a speed of rotation and aperipheral speed, the problems due to the limits of the strength of theequipment usually mean that the highest peripheral speeds are associatedwith the lowest speeds of rotation, and vice versa. This procedure isnot necessarily optimization as regards the formation of the product butis a compromise to ensure that the installation has a reasonable life.

The output of the material through each of the spinner orifices is avery important factor in this type of process. Of course this outputdirectly determines the total production capacity of the spinner. Lessobviously, this output per orifice is also a factor which substantiallyinfluences the characteristics of the fibers produced.

It will readily be seen that the greater the output per orifice, themore intensive the attenuation must be to obtain a constant fineness.

In light of these practical production requirements, the output perorifice cannot be very low. For a material comprising glass, or asimilar material, an output of 0.1 kg/day may be considered as a bottomlimit beyond which it is uneconomical to operate. Conversely, anexcessive output does not enable very fine fibers to be obtained. Forthe qualities required of an insulating material the material output ispreferably not more than 3 kg/day. Advantageously, the compromisesbetween the production capacity and the quality of the fiber result inthe output per orifice being controlled to a value of between 0.7 and1.4 kg/day.

Under the operating conditions of the invention, the production capacityof the spinner may be increased to very considerable values without thequality of the fibers or of the end product being reduced. Of course itis also possible to keep the production at a lower level, moreparticularly in order to improve the quality still more.

The production increase, which is an important economic factor, is ofparticular advantage. It has been the subject of numerous proposals inthe prior art: an increasing number of spinner orifices, an increasedorifice section, increased material fluidity, and so on. These variouselements effectively modify the output of material through the spinner,but the proposals made in this connection have resulted in a reductionin quality or difficulties in respect of the life of the equipment.

Thus an increase in the density of the orifices at the periphery of thespinner, i.e. an increase in the number of orifices per unit area, notonly weakens the spinner but, beyond a certain threshold, appears tohave an unfavorable effect on the fiber quality. It may be assumed thatthe primary fibers, which are very close to start with, collide andstick together before their attenuation is complete. This may be why thefibers collected under these conditions are less uniform, less fine andshorter.

A problem of the same kind is encountered when--again to increaseproduction capacity--the orifice section is increased while maintainingidentical viscosity conditions of the material formed into fibers. Inthis case the primary fibers are coarser and it is difficult to obtainsatisfactory drawing even if the conditions of the annular attenuatinggases are modified.

An increase in the fluidity for a given material, i.e. an increase inits temperature, gives rise to other problems. The operatingtemperatures are usually at the limit of what the spinner alloy canwithstand. The nature of the composition to be formed into fibers isalso often selected so as to allow for this type of limitation. It isimpossible to increase the temperature on this hypothesis unless thespinner is made from precious metals, and this gives rise to otherdifficulties, particularly in respect of costs, but also mechanicalcharacteristics.

An increase in fluidity can also be obtained by using materials of acomposition such as to achieve this result, but these compositions havethe disadvantage of being substantially more expensive.

Accordingly, prior art methods selected to improve the fiber qualitygenerally resulted in reduced productivity.

With the present invention, although there is still a quality/quantitybalance to resolve, the result is at a much more satisfactory level.Thus if good quality production is the objective comparable to thatproduced previously with centrifugal processes, a production on theorder of 12 tons of fiber material per day per spinner is readilyexceeded.

In practice, it appears advantageous according to the invention tomaintain production at a level above 17 tons per day, and production of20 to 25 tons or more can usefully be achieved while maintaining aproduct of excellent quality.

It is noteworthy that these improved results can be achieved without theuse of the spinner modifications discussed above. There is no need tochange the orifice density nor the orifice dimensions etc. The primaryfibers formed are therefore as a whole comparable to those of thesimilar prior art processes, but since fiber attenuation is improved,the quality of the product is better.

Attenuation of the fibers by the currents of gas is carried out by meansof a blast generator, the annular orifice of which is situated in theimmediate vicinity of the spinner. The gas flow is of a certain width sothat the filaments projected from the spinner remain fully immersed inthis flow of gas and are therefore kept under the conditions adapted totheir drawing.

The width of the flow at the start actually depends on the exactgeometric configuration of the fiber-forming system. For a givenarrangement, the useful variations are relatively limited inasmuch as toreduce the energy expenditure it is necessary to reduce the width of thegas flow as much as possible.

In conventional systems the width of the flow of gas is of the order of0.3 to 2.5 cm.

The pressure of the gas emitted under these conditions is between 100and 1,000 mm water column for an emission width less than 2 cm, and ispreferably from 200 to 600 mm water column for an emission width lowerthan 1.5 cm.

In analyzing hereinabove the respective effects of the peripheral speed,the material output, and the pressure of the drawing gases, it isnecessary to point out that in practice these various parameterstogether affect the quality or cost of the resulting products.Relationships can be drawn between these various parameters to allow forthe most advantageous operating conditions proposed by the invention.

Also, independently of these operating conditions, these relationshipsmust include an index of the quality of the product obtained. In thisway these relationships enable a clear distinction to be made betweenthe operating conditions according to the invention and those of theprior art.

Thus, according to the invention, the peripheral speed v of the spinnerorifices, expressed in meters per second, the mass of material q passingthrough each orifice in the spinner, expressed in kg per orifice and perday, and the pressure of emission p of the drawing gas expressed in mmwater column for an emission width of at least 5 mm and maximum 12 mmare so selected that the ratio qv/p is between 0.07 and 0.5 andpreferably between 0.075 and 0.2. It is noteworthy that the numerator ofthis ratio includes factors the increase of which tend to spread thefiber veil while the denominator recites a factor the increase in whichtends to restrict the veil. A better "umbrella" fiberization and ahigher quality fiber can accordingly be expected with high values of theqv/p ratio.

At this point in the description and before examining the examples ofimplementation and of products obtained, it is necessary to specify theparameters which characterize the products in order to better understandthe nature of the improvements brought by the invention.

For this purpose, the production of insulating materials constituted bya fiber blanket or mat will mainly be considered. These products alonerepresent a considerable part of all the applications of mineral fibers.Of course, that does not exclude the implementation of the invention forthe preparation of products intended for other uses.

As already stated, the two main properties required are thermalinsulating properties and mechanical properties. The latter relate toquite specific aspects of the insulating products. In addition, theproduct should lend itself to the handling and packing techniquesrequired of voluminous products of low specific gravity.

For a given product, the principal insulating property is defined by itsthermal resistance R. This expresses the capacity of the product inquestion to withstand heat changes when the product is subjected todifferent temperatures on either side. This value depends not only onthe characteristics of the fibers and their arrangement within theproduct, but also on the product thickness.

The thicker a product, the greater its thermal resistance. If,therefore, the thermal resistances of different products are to becompared, the thickness of these products must be specified. We shallsee hereinafter that the question of thickness is closely bound up withthe mechanical properties of the products. The literature alsooccasionally refers to thermal conductivity, a magnitude whichdisregards the product dimensions. It is to some extent an intrinsicmeasurement of the insulating quality. In practice, however, it is thethermal resistance that is most frequently used to characterize theproducts. It is therefore this magnitude that we use in the examples.Despite numerous studies carried out on this subject, it is stillimpossible to establish a complete correlation between the thermalresistance, or the thermal conductivity, and the measurable dataconcerning the structure of the product. Nevertheless, someconsiderations enable the production to be oriented according to therequired result.

Thus a larger quantity of fibers for a given covered area enables theinsulating properties to be increased, but this increase is accompaniedby an increasing cost. Wherever possible, therefore, it is advantageousto have a given thermal resistance with a minimum number of fibers. Acomparison of the specific weight, i.e. the mass of fibers per unit areafor different products of the same thermal resistance, gives ameasurement of their respective qualities. The quality of the product inthis case varies in inverse proportion to the specific weight.

It has also been established that the fineness of the fibers is animportant factor in respect of the formation of the insulatingqualities. For a given specific weight, the insulating properties of aproduct are all the greater the finer the fibers.

The mechanical magnitudes of use for an assessment of the product areessentially bound up with the problem of its packing. The reason forthis is that the products are very bulky and are advantageously storedin the compressed state. Nevertheless, the compressed product mustresume a certain bulk once it has been released in order to develop itsinsulating qualities to the maximum.

Before compression, the fiber mat itself assumes a certain thicknesswhich is different from the thickness before compression and from thethickness in the compressed state. It is desirable that the thicknessresumed after unpacking should be as large as possible to obtain aproduct having good thermal resistance.

More specifically, for an unpacked product of standardizedcharacteristics, and particularly of guaranteed thickness, it isimportant that it should occupy a minimum volume in the compressedstate. This guaranteed thickness resumption is also expressed by theadmissible compression ratio.

It is difficult to establish a close relationship between the mechanicaland the insulating qualities of a product, even if the attempt is madeto analyze them in terms of structure, fiber dimensions, and so on.Experiment simply shows that the improvement of the insulating qualtiesis compatible with the improvement of the compression ratio.

1. Comparative tests

These comparative tests were carried out to show the advantage of usingthe operating conditions specified by the invention as compared with theprior art operating conditions. Although a variety of factors are likelyto influence the quality of the final product, as far as possible, anattempt has been made in these tests to find operating conditions whichenable the parameters sensitive to various influences to be keptconstant.

The following table summarizes the various characteristics and theresults obtained in respect of the prepared product. Tests I, II and IIIwere carried out under operating conditions not equivalent to theinvention, while test IV was carried out under conditions in accordancewith the invention.

    ______________________________________                                                     I     II       III      IV                                       ______________________________________                                        Peripheral speed (v)                                                                         38      38       38     56.5                                   Output per orifice (g)                                                                       1.1     1.30     1.16   1.1                                    (kg/day)                                                                      Pressure (p) (mm water                                                                       758     1020     650    470                                    column)                                                                       qv/p           0.055   0.048    0.068  0.13                                   Micronaire fineness at                                                                       3       3        4.5    3                                      5 g (F)                                                                       Total centrifuge output                                                                      9       18       18     18                                     (tons per day)                                                                Specific weight for                                                                          948     1080     1215   945                                    R = 2 (g/m.sup.2)                                                             Compression ratio                                                                            4       2        4      4                                      Tensile strength                                                                             250     150      250    250                                    (g/g)                                                                         ______________________________________                                    

The quality of the products prepared is defined in this Table by thespecific weight required for a given thermal resistance for a likewisegiven thickness. As a reference we chose the thermal resistance of 2 m²°K/W measured at 297° K. for a thickness of 90 mm. These conditions areequivalent to a standard insulant.

The compression ratio is the ratio of the guaranteed nominal thicknessafter 3 months' storage in the compressed state to the thickness of thecompressed product.

The tensile strength is measured to the ASTM standard C 686 - 71 T.

Several conclusions can be drawn from the above Table.

For example, a comparison of test IV according to the invention and testII shows that for a constant product fineness and the same totalcentrifuge output, the conditions specified by the invention give abetter quality product (lower specific weight), a substantially improvedcompression ratio and better tensile strength.

For a given quantity of fiber material, the improvement found in thespecific weight enables the quantity of product to be increased by about10% or more. The improvement of the compression ratio gives a veryconsiderable saving in respect of transport and storage costs.

If, as in the case of Examples I and IV, the operating conditions areadjusted to have the same product qualities, it will be found that thecentrifuge output in the method according to the invention is increasedin considerable proportions, thus making the process decidedly morecost-effective.

Another way of comparing the invention with the prior art methods is toestablish conditions such that the output and mechanical qualities areidentical. In that case, as in Examples III and IV, the conditionsaccording to the invention given an appreciable reduction in the fiberfineness, showing that the fibers produced are finer, and that is whythe specific weight is lower for the same thermal resistance.

No matter how the procedure according to the invention is considered, itprovides an appreciable improvement over the prior art methods, asconfirmed by the results of the following tests carried out under otheroperating conditions.

2. Tests in respect of variations of various parameters

Various other parameters were modified according to the required productqualities while remaining in the conditions according to the invention.

The results of these tests are given below:

    ______________________________________                                                 V      VI      VII     VIII   IX                                     ______________________________________                                        Peripheral speed                                                                         56.5     56.5    75    56.5   71                                   (v) m/s                                                                       Output per orifice                                                                       1        0.9     0.7   1.25   1                                    (q) (kg per day)                                                              Pressure (p) (mm                                                                         160      325     270   440    300                                  water column)                                                                 qv/p       0.35     0.15    0.19  0.16   0.2                                  Total centrifuge                                                                         18       18      18    25     20                                   output (tons/day)                                                             Specific weight                                                                          1080     945     915   1140   865                                  for R = 2 (g/m.sup.2)                                                         Compression ratio                                                                        4        4       --    3      4                                    Tensile strength                                                                         300      250     --    250    250                                  (g/g)                                                                         ______________________________________                                    

Example V is an embodiment under the conditions of the invention toprepare a product comparable to that of Example IV. The mechanicalproperties are fully retained. The micronaire fineness goes from 3 to 4,i.e. the fibers are slightly less fine. The specific weight is thusslightly greater.

The type of production corresponding to Example V is advantageousalthough the products have a higher specific weight than those inExample IV, because under identical centrifugation conditions the changeof the fiber fineness is due to differences in the operation of theburner emitting the hot attenuating gases. The energy consumed foroperation of the burner in the case of Example V is considerably reducedby comparison with Example IV, and this can usefully make up for theincrease in specific weight.

Although it is impossible to give an exact comparison because the fiberfineness is not strictly the same in both cases, extrapolation showsthat for a 4.5 micronaire fineness the specific weight in the conditionsof Example V would be less than that of Example III. This confirms whatis apparent from a comparison of Examples II and IV, i.e., that thequality of the insulating products produced according to the inventionis better than that of the products obtained in the prior artconditions, even if this difference is greater for the finest products.

Example VI is a similar embodiment to Example IV but with the peripheralspeed maintained and the speed of rotation reduced. The properties ofthe products are identical except for experimental approximations.

Example VII is also interesting on the same lines. The influence of theperipheral speed on the quality of the prepared insulating product isagain clearly apparent from the specific weight.

Example VIII relates to a test for an output of 25 tons per day, whichdoes not represent a limit for the method according to the invention butwhich very clearly distinguishes it from the prior art systems in thisarea when good quality fibers are required. The micronaire fineness isstill relatively low and so is the specific weight, although slightlyhigher than Example V.

The remarkable increase in production thus obtained amply makes up forthis slight increase in specific weight required to achieve the desiredinsulating properties.

Example IX is another embodiment of the invention distinguished both bya high peripheral speed and a relatively low speed of rotation. Theseconditions enable excellent mechanical and insulating qualities to beachieved (the specific weight is particularly low) for a high centrifugeoutput.

3. Tests with constant qv/p ratio

These tests were carried out with a 300 mm diameter spinner having 11500 orifices.

The output per orifice was kept constant and at the same time theperipheral speed and the pressure of the burner were increasedapproximately in the same proportions to maintain a substantiallyconstant qv/p ratio.

The following table shows the results for four operating conditions:

    ______________________________________                                                     X     XI       XII     XIII                                      ______________________________________                                        Peripheral speed (v)                                                                         68      76       85    100                                     Output per orifice (q)                                                                       1       1        1     1                                       (kg per day)                                                                  Pressure (p) (mm water                                                                       410     460      510   600                                     column)                                                                       qv/p           0.165   0.165    0.166 0.166                                   Micronaire fineness at                                                                       4.0     3.5      3.0   2.5                                     5 g (F)                                                                       Total centrifuge output                                                                      11.5    11.5     11.5  11.5                                    (tons/day)                                                                    ______________________________________                                    

This Table shows that in the conditions according to the invention,variation of the variables v and p for a given unit output enables thefibers to be made finer in certain limits while at the same timeincreasing the peripheral speed and burner pressure, i.e. the speed ofthe gases. The result is a very substantial reduction of micronairefineness and hence the useful specific weight. It is also possible toobtain a micronaire fineness as low as 2 for a material output remainingrelatively high.

A similar experiment but holding the burner pressure constant shows thatthe unit output and hence the total output of the system must besubstantially reduced to obtain a very low micronaire fineness.

The same spinner was used in this series of tests.

    ______________________________________                                                   XIV   XV      XVI     XVII  XVIII                                  ______________________________________                                        Peripheral speed (v)                                                                       63      72      81    90    108                                  Output per orifice (q)                                                                     1.3     1.14    1.0   0.9   0.76                                 (kg per day)                                                                  Pressure (p) (mm water                                                                     410     410     410   410   410                                  column)                                                                       qv/p         0.20    0.20    0.20  0.20  0.20                                 Micronaire fineness at                                                                     4.5     4.0     3.5   3.1   2.7                                  5 g (F)                                                                       Total centrifuge                                                                           15      13      11.5  10.3  8.8                                  output (tons/day)                                                             ______________________________________                                    

The above two experiments, in the first of which the peripheral speedand the pressure are simultaneously increased and the second in whichthe peripheral speed is increased and the material output is reduced inorder to increase the insulating qualities of the product, do not givecompletely equivalent results.

It has been found that the mechanical qualities of the resulting matsare substantially better in the case of products obtained with a lowburner pressure, as indicated previously.

These differences are due to a better quality of the fiber produced fora relatively moderate drawing effect of the stream of gas as comparedwith that resulting from the rotation of the centrifuge.

In the foregoing examples we have shown, by certain comparisons,differences in respect of the manufactured products, and we have pointedout that these differences affect the production costs. It is difficultto exactly evaluate the economic gain provided by the inventionindustrially. Approximately, taking into account the various factorsinvolved, such as energy, product and equipment, the estimated savingmay be as much as 10% of the unit cost or more depending upon theconditions selected.

During these tests analyses were carried out to try to determine thefeatures of the fibers produced according to the invention as comparedwith fibers produced by the prior art in order to try to give details ofthe improvements found, particularly by evaluating the fiber finenessfor determination of the micronaire fineness. Apart from this indirectapproach it is difficult to determine the characteristics of the fibers,and particularly their length.

However, a number of findings lead to the conclusion that the operatingconditions according to the invention promote the formation of longerfibers. There is the improved tensile strength. There is also theability of the products to withstand a higher compression ratio and thethickness of the fiber mat leaving the receiving chamber before enteringthe thermal processing chamber for the binder. The conditions accordingto the invention give a substantial thickness increase, which may be asmuch as or more than 25% while the other conditions, more particularlymaterial output, remain identical. All these aspects to some extent maybe associated with an increase in fiber length.

With regard to the physical characteristics of the blanket or mat, thepresence of unfiberized particles in the mats is undesirable. Each oneof these particles has a significant material mass, when compared tothat of a fiber. Related to the material mass, the contribution of anunfiberized particle to the establishment of the insulating propertiesis quite inferior to that of a normal fiber. In other words, for acertain volume of fibrous material, a mat without unfiberized particlesis better insulation than a mat containing the unfiberized particles.

Furthermore, the presence of unfiberized particles is usuallyaccompanied by a profound lack of homogeneity of the characteristics ofall the fibers, another unfavorable factor for the quality of the matproduct.

For these reasons the absence of the unfiberized particles, or at leastvery low amounts, are particularly desirable, whatever fiber-productionmethod is considered In general, the techniques for production bycentrifugal blast attenuation, the spinner acting as a bushing, areknown to result in very low amounts of unfiberized particles. Thepresent invention has the ability to produce mats of which theunfiberized amounts are even lower.

By definition, unfiberized particles are qualified as all the particleshaving dimensions which are much greater than the average diameter ofthe fibers. The unfiberized particles are also characterized by a lowratio of length to the diameter. Usually, any particle having a diametergreater than 10 times the average diameter of the fibers can beconsidered as unfiberized.

The percentage of unfiberized particles in the products according to theinvention is less than 1% and preferably less than 0.80% by weight.

Still according to the invention, the percentage of unfiberizedparticles of which the dimensions are equal to or greater than 20 timesthe average diameter is less than 0.30% by weight of the entire mat.

The average diameter varies according to the nature of the productsprepared so that the size of the so-called unfiberized particles alsovaries. For the mats intended for insulation, and prepared by techniquessuch as those contemplated by the invention, the diameter of the fibersvaries from about 1 to 20 micrometers and, more simply, any particlewith a diameter of greater than 40 micrometers can be consideredunfiberized.

Following this definition, the mats according to the invention aredistinguishable from the mats previously obtained by analogoustechniques in that they contain at most 1% by weight of particles havinga diameter of greater than 40 micrometers. This amount is preferablyless than 0.08%.

The detail for the classification method of the fibers and particles isspecified further on in the examples.

Given that the measurement is made ponderally and that the unfiberizedparticles are heavier by nature than the fibers., the number ofunfiberized particles is very low.

In the products according to the invention, it is again appropriate tostate that the amount unfiberized is relatively small in relation tothose which are observed in products prepared according to othertechniques. The granulometric study shows in effect that the ponderalpercentage of particles of which the dimension is greater than 80micrometers is less than 0.3%.

In the products according to the invention practically no particlesexist having a diameter greater than 100 micrometers.

As indicated above, the absence of unfiberized particles is usuallyassociated with an excellent homogeneity in the fiber characteristics.This homogeneity can be determined, for example by the variations indiameter of the fibers. For example, the typical deviation in thediameter of the fibers is measured on the histogram of a mat specimen,that is, the difference between the extreme diameters, the interval ofwhich is 68% of all the fibers.

The coefficient of variation, that is the relation of the typicaldeviation to the average diameter for the specimen considered, is ameasurement of the dispersion of the fiber characteristics.

On this point also, the mats according to the invention compareadvantageously with the previous, known mats. The coefficient ofvariation is less than 0.85 and preferably less than 0.60 and can be aslow as 0.5. In other words, the histogram of the fibers in the matsaccording to the invention is very contracted.

A visual observation of the fibers in the mats according to theinvention shows a structural regularity substantially better than thatof the fibers in the traditional mats prepared by analogouscentrifugation techniques. The surface of the fibers seems smoother, thesection of the fibers is more constant, the deformed fibers--essentiallyre-glued fibers--are less numerous, etc . . . Of course, thesedifferences are difficult to quantify. They are, however, a usefulconfirmation of the noted advantageous structural characteristics of thefibers constituting the mats according to the invention.

The mats according to the invention are also distinguishable by thelength of the fibers from which they are formed. The direct measurementof the length of the fibers is an operation which comes up against greatdifficulties. The handling of the fibers causes ruptures which are asource of errors. Furthermore, only statistical data is significant.This necessitates the taking of many measurements.

To prevent these difficulties, an indirect method for the measurement ofthe length of the fibers consists of measuring a volume of sedimentationof the fibers suspended in water. The systematic studies conducted onthese types of measurements show that for a specifimen of a given mass,the volume of sedimentation increases with the length of the fibers fromwhich it is formed. In effect, it is understood that the aleatorydeposit causing the entanglement of the fibers is looser when the fibersare longer.

Furthermore, the volume of sedimentation is a function of the diameterof the fibers The finer the fibers, and therefore for a same mass theyare more numerous, the greater the volume.

In reduced-value areas it could be considered that the volume isapproximately proportional to the length and, inversely, proportional tothe diameter of the fibers.

This method is generally designated by the term "bulking". It isconveniently used and enables relative determinations of all the fibersof the specimen studied.

The measurements are made on fibers of which the length was previouslyreduced by limited grinding under well-determined conditions Thegrinding leads to fibers of a few millimeters length. However, thislength remains a function of the initial length.

The height or volume measurements of sedimentation are established inrelation to fibers of which the initial characteristics are known.Furthermore, the bulking method is also a way of judging the mechanicalstrength of the fibers in ratio to the grinding operation Of course,fragile fibers will lead to relatively low volumes of sedimentationafter the grinding, even if they are long at the start. This method ofmeasuring constitutes an overall appreciation of these two properties,namely, length and strength.

The considerable increase in the volume of sedimentation observed forthe mats according to the invention necessarily conveys a simultaneousimprovement of the length and strength of the fibers. The quality of thefibers forming the mats according to the invention was considered above.Obviously, the improvements stated concerning the fibers also influencethe properties of the mats, mechanical or insulating.

The comparisons with the known products are difficult to make due to thevery numerous parameters which contribute to the production of theseproperties. Traditionally, the insulating mats are classifiedcommercially as a function of their thermal resistance R. This thermalresistance is a function of the fineness of the fibers F, of the mass offibers per surface unit or "specific weight" m, but also of thethickness of the mat and of the fiber qualities indicated above.

The diminution of the specific weight in the case of the mats accordingto the invention indicates both the better regularity of the fibers andthe absence of unfiberized particles, but also, a better homogeneity ofthe fiber distribution in the mat.

To significantly define the specific weight of the mats, it is necessaryto specify on one side the thermal resistance of the mat R and thethickness e. These factors are interrelated by the expressions:

    m/R=m/e x λ

    m/R=m/e (A+Bρ+c/ρ)

    m/R=ρA+Bρ.sup.2 +C

In these expressions λ is the thermal conductivity and ρ is thevolumetric mass of the mat.

For the mats considered, the volumetric mass ρ being very low, the Bρ²factor can be disregarded at first approximation:

    m/R=ρA+C

When m is expressed by kg/m², R by m² :K/w at 297° K. and ρ by kg/m³ themats according to the invention are such that:

    m/R≦0.026ρ+0.2

Indicatively, for mats according to the invention of which the volumicmass is 10 kg/m³, the fibers have a fineness of 3 (under 5 g), for athermal resistance of 2 m².°K/w by order and the specific weight can beon the order of or less than 920 g/m².

The advantages associated with a lower specific weight in producing theinsulating mats are obvious. With the same volume of fibers, thequantity of mat produced is increased; the production cost is thereforelower.

The improvement of the mats according to the invention is alsonoticeable on the mechanical level. This is especially the case of thetensile strength which is measured according to the standard ASTM-C681-76. According to this standard, rings of specified size are cut inthe mat. These rings are placed on two cylindrical traction bars. Theyare subjected to increasing force. The force exerted at rupture ismeasured. To obtain comparable results, the force exerted is related tothe weight of the specimen.

The mats according to the invention bonded with a phenolic binderrepresenting at most 4.5% by weight of the finished product offer atensile strength preferably at least equal to 200 gf/g longitudinally,that is, when the rings are cut their length is extended in the samedirection as the mat product prepared.

The improvement of the tensile strength in relation to conventional matscan also be attributed to the presence of longer, more regular andbetter distributed fibers. In particular, it is understood that thelonger fibers improve the network structure of the mat. Furthermore, itis obvious that the fibers, of which the structure is more regular, andof which for this reason it can be thought that, individually, it has agreater strength, (the measurements made fiber to fiber, but in toosmall a number to be transposable to all the fibers of the mat, showedan increase in the tensile strength) lead to an improvement of thetensile strength of the mat.

The mats according to the invention as a result of the improvement oftheir mechanical properties are also adapted without difficulty to asignificant compression. In order to stock and transport thesevoluminous products of low mass, it is necessary, in effect, to havethem compressed. So that these mats then recover their good insulatingcapacity, they must be restored to their original thickness, even afterthree months in compression. The mats according to the invention cansatisfactorily undergo compression rates greater than or equal to 5 andwhich can be raised in certain cases to 7 or 8, thus showing that thehigh regular fibers of which they are formed lend themselves tosignificant deformation without breaking.

The mechanical properties of the fibers forming the mats according tothe invention also are indicated indirectly by the dust content of thesemats. Dust is defined by all particles not fixed to the mat and whichcan become detached from the latter without being subjected to effortsbeyond the framework of a normal implementation. The presence of dust inthe mat mainly results from the breaking of insufficiently strong fibersduring the conditioning of the mat (cutting, rolling, compressing, etc .. .).

The conditions of the dust measurement are described in the examples.These measurements significantly show a very low amount of dust for theproducts according to the invenrion. The amount of dust is less than 0.1mg/m². The absence of dust is an advantage to the user of the productand such a product is always preferred.

The regularity of the distribution of the fibers in a mat according tothe invention is important, and the mat should be as homogeneous aspossible. So that the insulation is effective, the mat must not havezones wherein the fibers are less abundant.

The fiber distribution can be determined by measuring the resistancewhich the mat presents to air circulation. The examples will show thaton this point also the mats according to the invention offer advantages.Especially for a volumic mass of 10 Kg/M³, the resistance measuredaccording to the standard ISO-DIS-4630 is greater than 4 Rayl/cm; for avolumetric mass of 20 Kg/m³, it is greater than 10 Rayl/cm.

To obtain fibers and mats having the properties indicated above, thecentrifugal blast attenuation is effected while maintaining theperipheral speed of the spinner at greater than 50 m/s. This speed canbe established with spinners of various diameters. To produce the fibersin economical quantities, it is preferable to operate with spinners ofwhich the diameter is substantially greater than 500 mm, preferably inthe range of 600-1500 mm.

Although the precise reasons for the improvement in the fibers and fibermat are not known, it is assumed that the fibers during their formationand for various reasons have less of a tendency to collide against eachother before being solidified. For this reason, the fibers are longeroverall and their properties are better. It is also possible that, underthe conditions followed, the forces acting on the fibers are moreregular and, as a consequence, the attenuation is developed moreprogressively with less filament ruptures or ruptures later on.

Regardless of the reasons, substantial improvements of the qualities ofthe products are effectively ascertained, as is shown in the followingexamples.

The following comparison is made with two commercial products,designated below by A and B. These two products are prepared like themats of the invention by centrifugation of the material forming thefibers through the orifices of a spinner and attenuation by a gascurrent.

The factors studied lead to the following results.

1. Determination of the Unfiberized Amount

The specimen analyzed is first rid of the binder which it contains. Itis subjected to a limited grinding in order to dissociate theunfiberized particles from the fibers to which they are ordinarilyattached.

The fibers are then separated from the unfiberized particles byentrainment in water in a separating column traversed by an ascendingcurrent of water.

The unfiberized particles are recovered at the bottom of the column.They are dried and sifted on vibrating screens arranged in series whichretain the unfiberized particles, the dimensions of which arerespectively greater than 100, 80 and 40 micrometers.

The results expressed in percentage by weight of the initial specimenare as follows:

    ______________________________________                                        Unfiberized Particles                                                                         A         B     Invention                                     ______________________________________                                        40 × 10.sup.-6 m                                                                        1.7       1.9   0.63                                          80 × 10.sup.-6 m                                                                        1.3       1.6   0.15                                          100 × 10.sup.-6 m                                                                       0.3       1.3   0.05                                          ______________________________________                                    

The mats according to the invention are distinguishable by the very lowunfiberized amount and also by the small size of the residualunfiberized particles.

The systematic errors introduced particularly by the method forseparating the fibers and the unfiberized particles are negligeable.

2. Histogram of the Fiber Diameters

Numerations are made on the specimens of the mats of example 1. Fromthese numerations, as a function of the diameter measured, thecorresponding histogram is established, then the average diameter, thetypical deviation and the coefficient of variation for each matspecimen. In these measurements of diameter, the unfiberized particlesare not taken into consideration.

The results are the following:

    ______________________________________                                                       A       B      Invention                                       ______________________________________                                        average diameter (10.sup.-6 m)                                                                 5.3       2.9    4.5                                         typical deviation (10.sup.-6 m)                                                                3.4       2.7    2.5                                         coeff. variation 0.64      0.93   0.5                                         ______________________________________                                    

The small extent of the typical deviation in the case of the inventionis especially remarkable in as much as the fibers on an average for thespecimen considered are finer than those of product A. This shows,indeed, a nicely contracted histogram. In other words, the fibers of themats according to the invention have a relatively constant diameter.

3. Estimation of the Lengths of the Fibers (Bulking)

The specimens of 5 g of unsized fibers, after passing through an oven at450° C., are ground for 10 seconds in 500 cm³ of distilled water.

After grinding, the aggregate of the water and the fibers is decanted ina graduated cylinder of 1000 cm³ and allowed to settle for 5 minutes(sedimentation practically achieved).

At the end of the sedimentation the top level of the column of fibers ismeasured in millimeters. These measurements are repeated at least threetimes to obtain an average value.

The results of these measurements are reported in the following table:

    ______________________________________                                                 A          B      Invention                                          ______________________________________                                        height after                                                                             41           128    150                                            3 min. (mm)                                                                   height after                                                                             39           127    149                                            5 min. (mm)                                                                   ______________________________________                                    

The preceding table shows that the products according to the inventionare formed from fibers substantially longer and/or more resistant thanthose of the analogous mats A and B.

4. Mass per Surface Unit

The specific weight or mass per surface unit of the various mats andtheir thermal resistance are set forth in the following table:

    ______________________________________                                                 A          B      Invention                                          ______________________________________                                        m g/m.sup.2                                                                              1000         1350   915                                            Rm.sup.2 ° K./W                                                                     2            2     2                                             m/R         500          675   457                                            ______________________________________                                    

The mats according to the invention require fewer fibers to produce thesame insulating quality.

5. Pull Resistance(ASTM-C-681.76)

The specimens are cut ring-shaped by stamping. Their dimensions are asfollows:

total length: 122 mm,

length between axes: 46 mm,

radius of the cavity: 12.5 mm,

outside radius: 38 mm.

Each specimen is weighed and then placed on a testing machine containingtwo cylindrical shafts 25 mm in diameter. The velocity of the movableshaft bearing the ring is 200 mm/min. The force at breaking is measuredand the ratio of this force to the specimen mass is determined.

The measurements are made for the cut specimens first longitudinally tothe mat, and then transversely.

The results are as follows, expressed in gf/g:

    ______________________________________                                                      A       B      Invention                                        ______________________________________                                        Longitudinal resistance                                                                       164       148    214                                          Transverse resistance                                                                         159       136    183                                          ______________________________________                                    

In both directions, the pull resistance of the mats according to theinvention appears superior to the comparative mats.

6. Dust Content

The measurement of the dust content is made on specimens of 800×400 mm.These specimens are suspended vertically from a vibrator on the insideof a cabinet. The vibrator frequency is 50 Hz and its range is 2.5 mm.The vibration is maintained for 2 hours. The particles which becomedetached from the specimen are recovered at the bottom of the cabinet.These particles are weighed and their mass is related to the opensurface of the mat sections.

Only the surface of the sections made in the thickness of the mat aretaken into account in this measurement. In effect, almost all of theparticles which become detached come from these surfaces In effect, theyhave undergone the hardest constraints, especially during the cutting.Furthermore, as a result of the smoothing effected during the treatmentof the mat in the oven, the top and bottom sides of the mat allowpractically no dust to pass.

For the mat A, an emission of 0.16 g/m² of section is found. This valueis only 0.07 g/m² for the mat according to the invention.

7. Air Permeability

The determination of air permeability, which is an indication of thefiber distribution, is measured according to the standard ISO-DIS-4638.

According to this standard, an outflow of air is created through a testtube containing a product specimen. The measurement of the drop inpressure between the two open ends of the tube measures itspermeability.

Measurements were made on products having two distinct volumetric masses(10 and 20 kg/m³). The results are set forth in the following table.They are expressed in CGS units by Rayl/cm.

    ______________________________________                                        kg/m.sup.3                                                                             A             B     Invention                                        ______________________________________                                        10       3             3     4.8                                              20       9             7.5   12                                               ______________________________________                                    

The good performance of the mats according to the invention areconfirmed by a simple observation. The fibers are arranged in awell-stratified manner and no zone appears having a lesser fibercontent.

Under these conditions and considering the fiber qualities noted above(regularity, absence of unfiberized particles, etc . . . ), the betterresistance to the passage of air appears quite coherent with the otherresults reported, particularly the improvement of the thermal resistancefor the same specific weight (or that which is equivalent, theobtainment of the same thermal resistance for a lower specific weight).

8. Pressure Resistance

Light mats, with thermal resistance of 2 m² °K/W. of which the nominalthickness guaranteed to the user is 90 mm, were compressed into rollform. In these rolls the thickness of the mat was only a fraction of thenominal thickness.

The maximum rate of compression (ratio of the nominal thickness to thethickness at the compressed state) which, after four months at thecompressed state enables recovery to the nominal thickness, wasdetermined for the products according to the invention and for productsA and B.

The results are the following:

    ______________________________________                                                  A         B     Invention                                           ______________________________________                                        Compression 4           4     5                                               Rate                                                                          ______________________________________                                    

The results show that the mats according to the invention have a higherpressure resistance. These results are even better when the specificweight of the product according to the invention is lower than thatdiscussed above.

We claim:
 1. A process for forming fibers from attenuable material which method comprises passing the material in an attenuable state through a centrifuge element equipped on its periphery with orifices through which the material is projected in the form of filaments, projecting a gas current at high temperature along the periphery of the centrifuge element transverse to the projection direction of the filaments to attenuate said filaments into fibers, rotating the centrifuge element at a rotational speed of between 800 and 6000 rpm and a peripheral speed of between 50 and 150 m/s, and maintaining the output per orifice between 0.1 and 3 kg per day.
 2. A process according to claim 1, characterized in that the peripheral speed comprises between 50 and 90 m/s.
 3. A process according to claim 1, characterized in that the rotation speed comprises between 1,200 and 2,250 rpm.
 4. A process according to claim 1, characterized in that the material output for each orifice g expressed in kg per day and the pressure at which the high-temperature gas stream is emitted p expressed in millimeters water column for an emission width greater than 5 mm and less than 12 mm are so selected that the ratio q.v/p is maintained in the range from 0.07 to 0.5, v being the peripheral speed in meters per second.
 5. A process according to claim 4, characterized in that the ratio q.v/p is maintained in the range 0.075 to 0.2.
 6. A process according to claim 4, characterized in that the output per orifice is between 0.7 and 1.4 kg per day.
 7. A process according to claim 4, characterized in that the high-temperature gas stream is emitted at a pressure of 100 to 1,000 mm water column for an emission orifice width not greater than 25 mm.
 8. A process according to claim 4, characterized in that the high-temperature gas stream is emitted at a pressure of 200 to 600 mm water column for an emission orifice width not greater than 15 mm.
 9. A process according to claim 4, characterized in that the speed of rotation is between 1200 and 2250 rpm.
 10. A process according to claim 9, characterized in that the output of fiber material from the centrifuge is greater than 12 tons of material per day.
 11. A process according to claim 9, characterized in that the output of fiber material from the centrifuge is equal to or greater than 20 tons of material per day. 